Such holding devices are used, for example, for lifting tools or transport equipment in order to move workpieces. U.S. Pat. No. 4,649,623 discloses a probe tip or stylus which can be held in arrangements of this kind and which are exchangeable in the probe head of coordinate measuring apparatus.
In the above-mentioned arrangements, the electromagnet weakens or intensifies the field of the permanent magnet during the exchange operation so that the part being held can be easily removed or can be pulled automatically onto the exchange face. The part being exchanged is held exclusively by the force of the permanent magnet in the time between the exchange operations.
The electromagnetic holding arrangements of the kind described above are usually configured in the manner disclosed in French Patent 73 03835 as well as East German Patent 127,065. According to these teachings, the permanent magnet is seated centrally in the bore of a pot-shaped flux-conducting part. The electromagnet is disposed ahead of the permanent magnets referred to the exchange face and is seated with its iron core in the flux-conducting part. A disc-shaped soft-iron part is disposed between the permanent magnet and the electromagnet in such a manner that a narrow gap remains between the outer side of the soft-iron part and the inner side of the pot-shaped flux-conducting part. This gap defines a short circuit path through which the flux of the electromagnets is guided so that it must not penetrate the permanent magnet.
This gap, however, defines a relatively high resistance for the flux of the electromagnet. The coil must therefore be essentially overdimensioned so that the field intensity required for neutralizing the flux of the permanent magnet can be achieved. This coil is relatively large and heavy. Furthermore, the coil needs relatively high electrical power which is transformed into heat and can introduce thermal problems especially for an application in measuring apparatus. Finally, the flux-conducting parts must extend with sufficient spacing in the region of the permanent magnet so that no further magnetic short circuit occurs. The volume of these known electromagnetic holding devices is therefore relatively large.
Japanese protective right 60-144914 discloses an electromagnetic holding arrangement comprising a flux-conducting part having an E-shaped section when viewed in cross section. The flux-conducting part has an annular gap in which the coil of the electromagnet as well as a permanent magnet is introduced. The poles of the permanent magnet are aligned so as to be radial referred to the axis of the magnetic field of the coil. The permanent magnet is mounted on the side of the flux-conducting part onto which the workpiece is pulled.
However, in this holding arrangement, the core of the soft-magnetic flux-conducting part is interrupted by a gap. A short circuit path extends across the base of the flux-conducting part behind the electromagnet and the gap interrupts this short circuit path for the flux of the permanent magnet so that the workpiece can be pulled into place exclusively by the permanent magnet without exciting the electromagnet. The electromagnet is only excited when the field of the permanent magnet is to be neutralized to loosen the workpiece.
The gap in the flux-conducting part however also defines a high magnetic resistance for the flux of the electromagnet. The same disadvantages mentioned above apply to this known electromagnetic holding arrangement insofar as the overdimensioning of the electromagnet is concerned.
The smallest possible weight and smallest possible assembly volume is desired if an electromagnetic holding device for exchanging a probe pin is to be built into the displaceable part of the probe head of a coordinate measuring apparatus. The total weight of the probe pin carrier including the weight of the probe pin itself and its holding device to be carried by the probe head has upper limits. For this reason, the weight for the probe pins is unavailable which was used for the holding device; that is, one is limited to a few relatively light probe pins. Furthermore, a high weight in the displaceable part of the probe head limits the dynamics of the coordinate measuring apparatus; more specifically, the permissible acceleration with which the probe head can be driven is limited. A tilting moment acts on the movable part of the probe head during acceleration and deceleration of the machine. This tilting moment must be held to a value which is so small that this movable part is not lifted out of its bearing and unwantedly simulates a touching. In addition to weight, the length of the probing device to be exchanged is a determining variable. A high weight for the holding device therefore limits the length of the probing devices which can be exchanged.